1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a metal separator and a membrane electrode assembly including a pair of electrodes and an electrolyte membrane interposed between the electrodes. Further, the present invention relates to a metal separator for the fuel cell.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. An oxidizing gas such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In the fuel cell, for example, a metal plate is used for fabricating the separator. The strength of the metal separator is high in comparison with a carbon separator, and the metal plate is suitable for fabricating a thin separator. The metal separator, which defines reactant gas flow fields having the desired shape, is fabricated by press forming in order to reduce the thickness of the metal separator, and reduce the overall size and weight of the fuel cell.
A resin seal is provided on the metal separator for electrical insulation and sealing. For example, in a production method disclosed in Japanese Laid-Open Patent Publication No. 11-309746, as shown in FIG. 7, a thin metal plate 3 is supported between surfaces of a fixed die plate 2a and a movable die plate 2b of an injection molding die 1. In a mold cavity, a resin channel 4 is formed in a marginal region of the thin metal plate 3, i.e., a cross sectional part 3a. 
Liquid silicone resin 6 is injected into the mold cavity from a gate 5 of the thin metal plate 3 to form a silicone resin layer 7 which covers the marginal region on both surfaces of the thin metal plate 3.
Normally, the thin metal plate 3 has a corrugated surface defining the reactant gas flow field and an orifice in the outer marginal region for improving the rigidity. The corrugated surface and the orifice are fabricated by press forming. In these portions, dimensional variation is likely to occur at the time of fabrication. Thus, for example, when the silicone resin layer 7 is formed integrally on the thin metal plate 3, the position of the orifice may be deviated from the position of the silicone resin layer 7 due to dimensional variation, and the metal portion around the orifice may be exposed to the outside to cause insulation failure. Consequently, the yield rate of the metal separator is lowered. In this case, the production cost is high, and the metal separator is not produced economically.